The role of breast cancer resistance protein in acute lymphoblastic leukemia.

PURPOSE Overexpression of the transporter ABCG2, also known as breast cancer resistance protein and mitoxantrone resistance protein, can confer resistance to a variety of cytostatic drugs, such as mitoxantrone, topotecan, doxorubicin, and daunorubicin. This study analyzes the ABCG2 expression and activity in 46 human de novo acute lymphoblastic leukemia B- and T-lineage (ALL) samples. EXPERIMENTAL DESIGN ABCG2 expression was measured flow cytometrically with the BXP-34 monoclonal antibody. ABCG2 functional activity was determined flow cytometrically by measuring mitoxantrone accumulation in combination with the ABCG2 inhibitor fumitremorgin C (FTC). To determine a possible effect of the transporters P-glycoprotein and multidrug resistance-associated protein (MRP1 and MRP2) on mitoxantrone accumulation, the accumulation was investigated in the presence of the P-glycoprotein inhibitor PSC 833 and MRP inhibitor MK-571. The ABCG2 gene was sequenced to investigate the amino acid at position 482. RESULTS In B-lineage ALL (n = 23), the median BXP-34:IgG1 ratio was higher, namely 2.4 (range, 1.7-3.7), than in T-lineage ALL (n = 23; 1.9; range, 1.2-6.6; P = 0.003). The addition of FTC to mitoxantrone treatment caused a median increase in mitoxantrone accumulation of 21% (range, 0-140%) in B-lineage ALL. In T-lineage ALL, this FTC effect was less pronounced (5%; range, 0-256%; P = 0.013). The influence of FTC on mitoxantrone accumulation correlated with ABCG2 protein expression (r = 0.52; P < 0.001; n = 43). The increase in mitoxantrone accumulation, when FTC was added to cells treated with both PSC 833 and MK-571, correlated with the ABCG2 expression in B-lineage ALL but not in T-lineage ALL. Sequencing the ABCG2 gene revealed no ABCG2 mutation at position 482 in patients who accumulated more rhodamine after FTC. CONCLUSIONS This study shows that ABCG2 is expressed higher and functionally more active in B-lineage than in T-lineage ALL.